1. Field of the Invention
This invention relates to an automatic exposure control means, and more particularly to an automatic exposure control means for a photographic or cinematographic camera in which the diaphragm thereof is operated by a servomotor which is controlled by a photoelectric exposure measuring circuit to obtain a correct exposure.
2. Description of the Prior Art
It has heretofore been known in the art to control the exposure in a camera by controlling the diaphragm by use of a servomotor connected with a bridge circuit which includes a photodetector as one of the four resistors to be balanced. In such a control means, the imbalance output of the bridge circuit is amplified and used for operating the servomotor.
The above-described exposure control means using a servomotor is generally apt to cause a vibration around the balance point of the bridge circuit owing to the inertia of the motor, friction resistance in the movable mechanism and a large variation in the driving current.
Since the vibration as mentioned above is undesirable from the viewpoint of automatic exposure control, it has been proposed to connect a braking coil to the servomotor separately from the driving coil so that the revolution speed of the servomotor may be detected by the braking coil and the revolution of the servomotor may be braked thereby to eliminate said vibration of the servomotor.
The above-described exposure control means using a servomotor with a braking coil has defects in that the variation in the current flowing through the coil corresponding to the variation in the brightness of the object is mixed and detected with the variation in the revolution of the servomotor, and accordingly a particular type of servomotor having a built-in braking coil must be used.